In deep hole drilling, it is traditional to refer to a high ratio between the aperture depth and the aperture diameter. Originally, the term "deep holes" referred to aperture depth of over five times the diameter. Today, the term "deep hole drilling" is used to describe methods for the machining of both short and deep holes, while it is the only method for drilling aperture lengths of more than ten times the diameter, it can also be competitive for short apertures down to two times the diameter because of its high metal removing capacity and precision. As in all metal machining operations, it is important that the material chips be broken and transported away without jamming and without effecting the drilled surface. Three different systems have been developed that permit trouble free machining of apertures depths of more than 100 times the diameter.
The first system is referred to as a gundrill system, where the cutting fluid is supplied through a duct inside the drill and irrigates the cutting edge, after which the cutting fluid removes the chips through a V-shaped chip flute along the outside of the drill. The cross-section of the tube occupies 3/4 of its circumference due to the V-shaped groove or flute, and sometimes the system is referred to as a 3/4 drill system. Gundrill systems can be seen in U.S. Pat. No. 5,181,812, U.S. Pat. No. 4,726,717 and Great Britain Patent Specification 1,435,166.
The second system is referred to as a single tube system which is based on external cutting fluid supply and internal chip support. Cutting fluid is typically supplied through the space between the drill tube and the drilled aperture. The cutting fluid is removed along with the chips through the central passage of the drill tube. Chip transport through the tube occurs at a relatively high velocity. The cross-section of the shank or tube can be made completely round, which provides much higher rigidity than in the gundrill system.
The third system is referred to as the double wall or ejector system where the cutting fluid is pumped to the cutting face through the space between an inner and an outer tube. This configuration eliminates the need for a high pressure seal between the workpiece and the drill bushing. At least a portion of the cutting fluid is delivered to the drill head, where it is forced through a number of apertures to lubricate and cool cutting edges of the drill head. An example of an ejector system can be seen in U.S. Pat. No. 3,511,120.
Deep hole drilling machines can also be broken down into two main groups, single cutting edge tools, sometimes referred to as single lip, end cutting tools, and multiple edge cutting tools, sometimes referred to as multiple-lip cutting tools. All gun-type tools are single lip, end cutting tools, incorporating bearing pads to support and guide the tool. When a second lip is added to a gun-type tool, the guidance principle changes, and the tool becomes a multiple-lip high pressure coolant tool rather than a gun-type tool. For a given workpiece material, the same factors effecting chip formation with gun-type tools (rotational speed, longitudinal feed, nose or point angle and coolant pressure and flow rate) effect chip formation with multiple-lip tools. Multiple-lip tool applications are best suited for brittle materials that produce powder, grain or sliver chips. These brittle materials may include cast metals, such as cast magnesium, aluminum, iron, brass and bronze, as well as non-metals such as carbon, graphite, certain woods and plastics. Multiple-lip tool applications are not well suited for ductile materials producing stringy chips where the chip removal is from the rear of the shank. For these applications, typically gun-type tools have been recommended.
The types of chips formed in a given workpiece material by a multi-lip tool can be controlled within a limited range. Increasing the tool rotational speed makes the chips thinner. The simplest way to reduce chip size when using multiple-lip tools is to increase the longitudinal feed rate, making the chips thicker so that they will break rather than curl. If this fails to solve the problem, the next typical corrective measure is to change the geometry of the tip. By increasing the outside angle, the point makes a deeper crease in the chip, possibly splitting the chip lengthwise. As a last resort, chip breakers are typically added on multiple-lip tools similar to those used on gun-type tools. However, chip breakers are often more difficult to add to multiple-lip tools. The coolants have the same function in chip formation and control when using multiple-lip tools as the coolants do in gun-like tools. High quality, light viscosity coolants let the chips slide freely off the cutting edge, making the chips thinner and possibly allowing increased feeds.
A multiple-lip internal chip removal drill is similar in design to the internal chip removal gundrill, but has a cutting edge divided into three sections. The three cutting edges are located on a straight line across the center of the drill. One cutting edge starts at the periphery of the tip and cuts through approximately 40% of the radius. Another cutting edge cuts through the center, and approximately 40% of the radius on the same side of center as the peripheral cutting edge. The third cutting edge is located in the center of the radius 180.degree. from the other two edges and partially overlaps the cutting area of the other two cutting edges. This three-lip arrangement, with cutting edges on both sides of center, gives the tool partially balanced cutting forces, and takes some of the load off of the bearing pads. The bearing pads are located at about 90.degree. and 180.degree. from the peripheral cutting edge. The chips exit through two chip mouths, located on each side of center. The chips are forced into the chip mouths by high pressure coolant, which may be forced around the outside of the tool by a fluid transfer unit for a single tube system or forced between the outer tube and the inner tube for a double wall or ejector system.
Deep hole drilling machines are often designed with a rotating workpiece, a rotating tool or both a rotating workpiece and a rotating tool. When machining asymmetric workpieces, a rotating drill and a non-rotating workpiece is typically provided, since the workpiece cannot rotate at sufficient speed. When boring long, slender workpieces, a non-rotating drill is typically fed into a rotating workpiece. If a high tolerance bore is required, both the drill and the workpiece may rotate with the drill rotating in an opposite direction from that of the workpiece.
While double wall or ejector drilling and gundrilling have been adapted for configurations with rotating tooling and stationary workpieces, a satisfactory configuration of a single tube system with rotating tooling and stationary workpieces has not been achieved. The prior known gundrills, where cutting fluid is supplied through a passage in the drill shank to flush chips out away from the drill head through a V-shaped trough in the drill shank, can drill a 78 inch deep bore of 0.906 inch diameter in approximately 1 1/2 hours.